As is well known, a plurality of electronic control units (ECUs) mounted in a vehicle usually compose a vehicle network system. These ECUs are network-connected to one another, thereby making it possible to mutually exchange respective pieces of information (vehicle information) of the ECUS. An example of a communication system composing such a vehicle network system is a controller area network (CAN).
By virtue of the CAN, a plurality of ECUs that share a bus, i.e., a communication line, are able to cause a message to flow in a bus, by making determinations by themselves. Accordingly, it is easy to transmit a message to the bus from each ECU. Therefore, for example, if an invalid ECU is connected to the bus of the CAN, an invalid message is likely to be transmitted to the bus. The ECU that has received such an invalid message is likely to process the invalid message in the same manner as a normal message.
Conventionally, a technique for preventing communication provided by an invalid message has also been developed, an example of which is disclosed in Patent Document 1.
The technique disclosed in the Patent Document 1 ensures secrecy of communication data between a master wireless communication device and a slave wireless communication device. The technique uses a secret key shared between the master wireless communication device and the slave wireless communication device. Using a secret key, the master wireless communication device encrypts a self-created encryption key and transfers this encryption key to the slave wireless communication device. Using a common secret key, the slave wireless communication device decrypts the transferred encryption key. Thus, the encryption key is shared between the master wireless communication device and slave wireless communication device. The foregoing technique uses the shared encryption key to encrypt and decrypt communication data exchanged between the master wireless communication device and slave wireless communication device.